In present-day hybrid video encoding schemes, the images are encoded in two phases, a first phase of prediction by motion compensation followed by a second phase of encoding of prediction residues.
Motion compensation techniques include the known technique of backward motion compensation, an example of which is illustrated in FIG. 1A. Backward motion compensation enables the prediction of an image Ic, from at least one reference image Ir in taking account of the shift vectors pointing from the current image to one or more reference images.
This prediction by backward motion compensation comprises two main steps:                the image to be predicted Ic is divided into a set of blocks;        for each block of this image to be predicted Ic, a prediction is made by means of a shift vector pertaining to a shift relative to the reference image Ir.        
Thus, any point or pixel of a considered block of the image to be predicted Ic is assigned the value of the point of the corresponding reference image, shifted by the value of the shift vector associated with the block considered. This technique makes it possible to provide a prediction value for each point of the image to be predicted.
Motion compensation techniques also include what is known as “forward” motion compensation, an example of which is illustrated in FIG. 1B. Forward motion compensation enables the prediction of an image Ic from at least one reference image Ir in taking account of the shift vectors pointing from one or more reference images Ir to the current image Ic.
This prediction by forward motion compensation comprises two main steps:                the reference image Ir is divided into a set of reference blocks;        for each reference block of the reference image, a shift vector is set up and for each point of this reference block, the point of the image to be predicted Ic is assigned the value of the point of the reference image, shifted by the shift vector.        
One drawback of this technique of forward motion compensation is that it gives rise to the appearance of overlap zones when several blocks overlap one another, these overlap zones being referenced R in FIG. 1B. Furthermore, the use of different shifts on the forward projected blocks also causes the appearance of holes zones between the blocks, these holes being denoted as D in FIG. 1B.
The absence of an assigning of values in the holes limits the performance of the proposed encoding scheme.
A solution has been proposed in the French patent application FR 2 917 872 filed on behalf of the present Applicant.
This technique relies on the use of motion tubes to represent the sequence of images. These motion tubes move in space in the course of time, following the paths of motion of the sequence. This “temporal persistence” of the tubes provides for a continuous representation of the video sequence and the efficient representation of the zones of both continuity and discontinuity of the motion of a sequence.
For a given motion tube, initialized in a reference image, the prediction of an image to be predicted belonging to this same tube is obtained by forward projection of each block of the reference image by means of one or more motion vectors.
One drawback of this technique is that prediction by forward motion compensation of an image to be predicted of a motion tube uses only the reference image in which the motion tube is initialized. Now this reference image can be very distant from the image to be predicted. Consequently, the efficiency of prediction is all the more limited as the reference image is distant from the image to be predicted.
It is therefore necessary to provide novel image encoding/decoding techniques implementing prediction by forward motion compensation, enabling these prior-art techniques to be improved.